This invention relates to the purification of TNT (2,4,6-trinitrotoluene, also called .alpha.-TNT) for military use through a process of removing the unsymmetrical 2,4,5- and 2,3,4-isomers from TNT (containing approximately 4% unsymmetrical isomers) conventionally manufactured by nitration processes from toluene and mixed nitric and sulfuric acids.
Standard industrial procedures for removing these isomeric impurities are to treat the crude TNT with hot aqueous sodium sulfite ("sellite") which reacts with the reactive nitro groups in the meta(3) position and produces a waste sellite solution containing a mixture of sodium 2,4-dinitrotoluene-3 and 5 sulfonates. This process, however, results in a very concentrated and intensely reddish colored solution, the disposal of which is both costly and a serious water pollutant.
Purification of TNT has also been accomplished by treatment of crude TNT with aqueous ammonium sulfite and ammonium bisulfite through the process described in U.S. Pat. No. 3,956,409 to Everett Gilbert, heating crude TNT with an aqueous mixture of magnesium sulfite and magnesium bisulfite as described in U.S. Pat. No. 4,003,953 to Everett Gilbert and through purification of crude dinitrotoluene intermediate by extraction with C.sub.3 -C.sub.8 alkanes prior to nitration, as described in U.S. Pat. No. 3,949,008 to David H. Rosenblatt et al. These procedures, while achieving acceptable levels of isomer removal, have still presented problems of disposal and have generally been time-consuming, uneconomical processes.
One procedure for purifying TNT which has been considered promising is the recrystallization of TNT from nitric acid used by A. B. Bofors in Sweden, as described in their U.S. Pat. No. 2,874,196. A major problem which has limited the widespread adaption of the nitric acid crystallization process is the disposal of the intendant alpha TNT and unsymmetrical isomer mixture obtained in the filtrate from the recrystallization. This mixture, known as "Isotrioil," has been used to some extent in commercial dynamite production, but this is not an economically viable mode of disposal in this country where supply of Isotrioil far exceeds commercial demand. Destruction of the Isotrioil by burning is technically feasible, but is also an uneconomical solution since at least half of the mixture comprises the desired alpha isomer. Proposed recover of alpha isomer by a "sweating" procedure is both time-consuming and yields an impure product.
The instant invention provides for purification of Isotrioil by adsorption and removal of the unsymmetrical 2,4,5- and 2,3,4-isomers by passing a solution thereof in a developing solvent system comprising polar and non-polar organic solvents, such as 1,1,2-trichloro-1,2,2-trifluoroethane and acetonitrile, through a column packed with silica gel.